As a conventional power conversion device, there is known a series-connected multi-level power conversion device that has a plurality of output phases each of which is constituted by serially-connected power conversion cells. The series-connected multi-level power conversion device includes a multi-winding transformer and a plurality of single-phase power converters. The multi-winding transformer transforms a multiphase alternating input voltage into a primary winding and outputs the transformed voltage to a plurality of secondary windings. The plurality of single-phase power converters are respectively connected to the secondary windings of the multi-winding transformer.
In the series-connected multi-level power conversion device, the secondary windings of the multi-winding transformer output voltages whose phases are different from one another in order to reduce a harmonic current in the primary winding. More specifically, there has been proposed a method in which, when the series-connected multi-level power conversion device includes nine single-phase power converters, secondary windings connected to three single-phase power converters that constitutes one of U, V, and W phases have a voltage phase difference of 20 degrees and all the secondary windings have a voltage phase difference of 20/3 degrees. This method has been known as disclosed in, for example, Japanese Laid-open Patent Publication No. 2008-295149.